1. Field of the Invention
The present invention relates to an exposure control method for a camera and an apparatus for the method.
2. Background Art
In the case of an ordinary negative film, even if a picture frame is not exactly taken at a proper exposure value, it is possible to correct the image by controlling print-exposure so as to make a reasonable print. However, in an instant camera using a self-developing film, it is impossible to correct exposure afterward. Therefore, it is very important for instant photography to expose the film at a proper value.
In view of this, an exposure control method, called mix-exposure method, is suggested, for example, in U.S. Pat. No. 4,530,585 and JPB 5-14256, wherein an amount of flash light is always automatically projected even to a bright subject. In this method, a program shutter is controlled to obtain 80% of a necessary exposure amount from ambient light, while the flash light is added to make up for the remaining 20% of the proper exposure value, on the assumption that a proper exposure value (Ev) is determined in correspondence with a light value (Lv). According to the known method, even though the exposure value is calculated based on an average brightness, which is more or less affected by a background or subsidiary subjects, a satisfactory exposure of the main subject may be achieved in most cases, because the brightness of the main subject, mostly the nearest subject, is supplemented with the flash light.
Recently, cameras have been known that divide a photographic scene into several areas when measuring subject brightness. An exposure value is calculated based on respective brightness values of these area after weighting each value with an appropriate coefficient. At that time, it is determined from the divisional brightness values, whether the scene is front-lighted or back-lighted. If the scene is back-lighted, an optimum exposure value can be preferably determined based mainly on brightness values of those area which correspond to a main subject. In day-light synchronized photography, it is possible to obtain an almost satisfactory exposure with respect to both a main subject and a subsidiary subject by controlling shutter speed and aperture value based on a background brightness and illuminating a main subject with a supplementary flash light.
However, to determine an optimum exposure value in the divided brightness measurement with more accuracy, it is necessary to increase the number of divisional areas and weight the respective brightness values by using a complex calculation in accordance with the brightness distribution pattern. Even such a complex and expensive method is not always able to provide an optimum exposure of the main subject, because of wide variety of actual photographic scenes.
Also in the known mix-exposure method, since the mix rate of flash light to ambient light is determined based on an exposure value derived from an average brightness value of the whole area of the photographic scene, consequent photo-prints may contain under-exposed main subjects especially when the main subject is back-lighted or distant.
To achieve a reasonable exposure for any scene with a simpler and economic method, a camera has been known which adopts a photometric device having a central brightness measuring element and a peripheral brightness measuring element for measuring a central brightness from a central area of the scene and a peripheral brightness from a peripheral area. Because the main subject such as a human subject is very often located in the center area of the scene, the two-area divided brightness measuring method facilitates discrimination between front-lighted and back-lighted scenes, and permits controlling exposure mainly in accordance with the central brightness value.
However, since the central brightness value has a large effect on the exposure control especially for back-lighted scenes in this method, it is important to design the central brightness measuring element so as to pick up light from the main subject at a high probability. If the central brightness measuring element is too small, it tends to miss the main subject. If the central brightness measuring element is too large, it tends to cover beyond the main subject to a larger extent. In either case, an optimum exposure value could not be derived from the central brightness value.
In view of the foregoing, an object of the present invention is to provide an exposure control apparatus using the two-area divided brightness measurement which is able to determine an optimum exposure value for a main subject at a high probability.
Another object of the present invention is to provide an exposure control method by which an optimum exposure value for a main subject can be determined for both front-lighted and back-lighted scenes without the need for complicated processing.
A further object of the present invention is to provide an exposure control apparatus which achieves an accurate exposure control even when the speed of shutter blades unexpectedly changes.
To achieve the above objects in a method using a central photometric element for detecting a central light value from a central area of a photographic scene and a peripheral photometric element for detecting a peripheral light value from a peripheral area of the scene, the present invention suggests calculating an exposure value for controlling exposure amount according to the following equation:
Es=log2{1xe2x88x92(1xe2x88x92xcex1)2LVbxe2x88x92LVa}xe2x88x92log2xcex1+LVa
wherein Es represents the exposure value, LVa and LVb respectively represent the central and peripheral light values, and xcex1 represents a correction coefficient which is determined depending on whether the scene is back-lighted or front-lighted. The central photometric element preferably has a trapezoid shape.
In a mix-exposure method according to a preferred embodiment of the invention, a central exposure value is calculated from the central and peripheral light values using a specific equation and a first specific correction coefficient, and a peripheral exposure value is calculated from the central and peripheral light values using a second specific equation and a second correction coefficient.
The values of the first and second correction coefficients are determined depending on whether the scene is back-lighted or front-lighted, more preferably, depending also on subject distance. Thereafter, an exposure value effective for controlling a program shutter is determined based on the peripheral exposure value and a bias value added to the peripheral exposure value for biasing the effective exposure value toward under-exposure side. On the other hand, percentage or mix rate of flash light is determined based on a difference between the effective exposure value and the central exposure value when the scene is back-lighted. For front-lighted scenes, mix rate of flash light is determined based on a difference between the effective exposure value and the peripheral exposure value.
According to another preferred embodiment, one of shutter blades of the program shutter is provided with signal elements for monitoring rotational position of the shutter blade and/or moving speed of the shutter blade. By starting clocking when the number of pulses detected from the signal elements reaches a set value determined in accordance with a proper exposure value, and outputting a shutter close signal when a time period, which is also determined in accordance with the proper exposure value, has been clocked, the effect of change in shutter blade moving speed on the exposure amount is minimized.